Aluminum Oxide

OVERVIEW

Aluminum oxide (uh-LOO-min-um OK-side) is white crystalline powder that occurs in nature in a variety of minerals, including boehmite, bayerite, corundum, diaspore, and gibbsite. Corundum is second hardest naturally occurring mineral. Only diamond is harder. Aluminum oxide occurs in a variety of chemical forms in a variety of gemstones, including chrysoberyl, ruby, sapphire, and spinel. The color of these gemstones is a result of impurities, such as chromium (in the case of ruby) and iron and titanium (in the case of sapphire). The colors may also vary depending on the kind and amount of each impurity.

Aluminum oxide's commercial uses depend not only on its hardness, but also on its high melting point and its low electrical conductivity. The compound is also non-combustible and resistant to attack by most solvents and other chemical agents.

KEY FACTS

OTHER NAMES:

Alumina

FORMULA:

Al₂O₃

ELEMENTS:

Aluminum, oxygen

COMPOUND TYPE:

Metallic oxide

STATE:

Solid

MOLECULAR WEIGHT:

101.96 g/mol

MELTING POINT:

2072°C (3762°F)

BOILING POINT:

2980°C (5396°F)

SOLUBILITY:

Insoluble in water and most organic solvents; slowly soluble in basic solutions.

HOW IT IS MADE

Aluminum oxide is produced by washing the rocky material bauxite with a hot solution of sodium hydroxide (NaOH). The aluminum hydroxide (Al(OH₃)) that is produced in this reaction is then heated to drive off water, producing aluminum oxide. Wastes from coal mining operation are also treated to extract the aluminum sulfate (Al₂(SO₄)₃) they contain. The aluminum sulfate is converted to aluminum hydroxide which, again, is heated to produce aluminum oxide.

COMMON USES AND POTENTIAL HAZARDS

The primary use of aluminum oxide is the manufacture of aluminum metal. When an electric current is passed through molten (melted) aluminum oxide, the compound breaks down to form aluminum metal and oxygen gas. The method is called the Hall process after the American chemist Charles Martin Hall who invented it.

Aluminum oxide is also widely used as an abrasive. An abrasive is a very hard material used to grind, polish, sand, scour, scrub, smooth or polish some other material. Among the products that include aluminum oxide as an abrasive are emery boards, sandpaper, grinding and polishing wheels and belts, lens grinding devices, and gem polishing wheels.

The high melting point of aluminum oxide also makes it a good refractory product. A refractory material is one that does not melt easily, making it suitable for lining the inside of furnaces or the manufacture of glass and ceramic materials that will not melt when exposed to very high temperatures. Some other uses of aluminum oxide include:

• In finely-divided form, as the packing material in chromatographic columns. Chromatography is a process by which individual components of a mixture are separated from each other by passing them through a tube filled with some absorbent material (such as aluminum oxide).
• As a catalyst in many industrial chemical reactions;
• In the paper-making process, as a filler that adds body to the final product;
• For the production of artificial gemstones;
• As a food additives, where it acts as a dispersant, a substance that keeps a product from clumping together in a package; and
• As the internal coating on frosted light bulbs.

Interesting Facts

• The hardness of materials is measured on the Mohs scale, named after the German mineralogist Frederick Mohs (1773–1839), who suggested the system. The scale runs from 1 (the softest known natural material, talc) to 10 (the hardest known natural material, diamond). Corundum has a hardness of 8.8 on the Mohs scale.
• The invention of a method for making aluminum metal from aluminum oxide by the 23-year-old American chemist Charles Martin Hall (1863–1914) in 1886 resulted in a reduction in the price of aluminum metal from about twelve dollars a pound to less than a dollar a pound.

The U.S. Occupational Safety and Health Administration (OSHA) classifies aluminum oxide as a nuisance dust in the workplace. A nuisance dust is one for which no serious harmful effects have been identified as long as its release is kept under control. It may, however, cause unpleasant symptoms, such as irritation of the skin, eyes, and lungs if present in unusually large amounts. The simplest treatment for problems related to nuisance dusts is to move away from the contaminated area where an adequate supply of fresh air is available.

Words to Know

CATALYST

A material that increases the rate of a chemical reaction without undergoing any change in its own chemical structure

CHROMATOGRAPHY

A process by which a mixture of substances passes through a column consisting of some material that causes the individual components in the mixture to separate from each other.

DISPERSANT

A substance that keeps another substance from clumping together or becoming lumpy.

REFRACTORY

Does not melt easily; able to withstand high temperatures.

SOLVENT

A substance that is able to dissolve one or more other substances.

FOR FURTHER INFORMATION

"Aluminum Oxide, Al₂O₃." Accuratus. http://www.accuratus.com/alumox.html (accessed on September 19, 2005).

Frost, Randall. "Abrasives." Gale Encyclopedia of Science. Edited by K. Lee Lerner and Brenda Wilmoth Lerner. 3rd ed. Vol. 1. Detroit, Mich.: Gale, 2004.

"The Mineral Corundum." Amethyst Galleries. http://mineral.galleries.com/minerals/oxides/corundum/corundum.htm (accessed on September 19, 2005).

Misra, Chanakya. Industrial Alumina Chemicals. Washington, D.C.: American Chemical Society, 1986.

See AlsoAluminum hydroxide